FIG. 11 shows basic workings of the optical disk device.
Optical disk 10 is a donut-shaped disk (disc) having a hole at the center, and tracks are created using trains of pits arranged concentrically on its signal recording plane (lower plane in FIG. 11) 10a. During playback, spindle motor 12 rotates optical disk 10 at a prescribed linear velocity by means of a CLV (Constant Linear Velocity) method, for example.
Movable optical pickup 14 is provided opposite signal recording plane 10a of optical disk 10 in the radial direction. Optical pickup 14 focuses and irradiates laser beam LB onto signal recording plane 10a of optical disk 10, detects a reflected beam of light from signal recording plane 10a, applies photoelectric conversion to it, and generates an electric signal having a waveform corresponding to the bumps of the pit trains. The electric signal output from optical pickup 14 is supplied to RF amplifier 16, and a regenerative data signal, a tracking error signal, and a focus error signal are generated by said RF amplifier 16. Regenerative data signal SM output from RF amplifier 16 is input to a signal processing part (not illustrated) and receives necessary signal processing, such as decoding and error correction. Tracking error signal TE and focus error signal FE output from RF amplifier 16 are sent to pickup servo 18. The pickup servo 18 performs tracking control for positioning regarding beam spot SP of laser beam LB on a track (pit train) or for following it and focusing control for focusing beam spot SP on the pits appropriately.
An example photo-detector serving as a photoelectric conversion part built into optical pickup 14 is shown in FIG. 12. The photo-detector is of a quadrant type for a push-pull system, wherein, for example, 4 photo-conversion parts configured with photodiodes or light receiving areas A, B, C, and D are arranged while separated from one another by assembling them from 4 diagonal directions. Beam LB′ of the returned light or reflected light from signal recording plane 10a of optical disk 10 is focused and incident to the light receiving areas (A, B, C, and D) of the photo-detector through a detection optical system, such as an optical lens, and electric signals (will be referred to as RF signals, hereinafter) SA, SB, SC, and SD are generated according to the respective quantities of light or the intensities of light received by respective light receiving areas A, B, C, and D. Furthermore, these RF signals are voltage signals applied with I–V conversion within pickup 14, and they use the level of reference voltage Vc given as a bias voltage to the pickup from the outside as a reference. Waveforms of RF signals SA, SB, SC, and SD are shown schematically in FIG. 13.
In said photo-detector, RF signals SA and SD obtained from light receiving areas A and D located on the left side of the center line in parallel with the direction of the track are of the same phase, RF signals SB and SC obtained from light receiving areas B and C located on the right side are also of the same phase, and RF signals SA and SD and RF signals SB and SC are of opposite phase.
When beam spot SP of laser beam LB is positioned at the center of the track (when on-track) on signal recording plane 10a of optical disk 10, reflected beam LB′ from signal recording plane 10a is incident onto the center parts of the light receiving areas (A, B, C, and D) of the photo-detector, and the levels of RF signals SA and SD obtained from light receiving areas A and D on the left and the levels of RF signals SB and SC obtained from light receiving areas B and C on the right become approximately the same.
However, as beam spot SP deviates from the center of the track in the radial direction, reflected beam LB′ is incident onto positions deviated either to the left or the right from the center parts of the light receiving areas (A, B, C, and D) of the photo-detector, so that the levels of RF signals SA and SD on the side of light receiving areas A and D on the left and the levels of RF signals SB and SC on the side of light receiving areas B and C on the right become different. For example, when beam spot SP deviates inward along the radial direction, the focus position of reflected beam LB′ is offset further toward the left side of the light receiving areas (A, B, C, and D), and the levels of RF signals SA and SD on the side of light receiving areas A and D on the left become higher than when they are on-track, while the levels of RF signals SB and SC on the side of light receiving areas B and C on the right become lower than when they are on-track. In contrast, when beam spot SP deviates outward in the radial direction, the focus position of reflected beam LB′ offsets further toward the right side of the light receiving areas (A, B, C, and D), and the levels of RF signals SB and SC on the side of light receiving areas B and C on the right become higher than when they are on-track, while the levels of RF signals SA and SD on the side of light receiving areas A and D on the left become lower than when they are on-track.
The basic configuration of a conventional tracking error detection circuit of a push-pull system is shown in FIG. 14. Said tracking error detection circuit has a pair of adding circuits 100 and 102, a subtracting circuit 104, and low-pass filter 106. Adding circuit 100 on one side takes the sum (SA+SD) of RF signals SA and SD from light receiving areas A and D on the left side of the photo-detector (FIG. 12), and the other adding circuit 102 takes the sum (SB+SC) of RF signals SB and SC from light receiving areas B and C on the right. Subtracting circuit 104 takes the difference {(SA+SD)−(SB+SC)} between the added signals. Low-pass filter 106 removes high-frequency elements, that is, RF signal elements, from the difference signal output from subtracting circuit 104 and outputs tracking error signal TE.
In the case of an optical disk device, the accuracy of the tracking error signal determines the precision of the tracking servo. Typically, when the optical pickup is to undergo jumping from the track currently being tracked to another track, if the tracking error signal is not accurate, a long time is required from when the tracking servo is activated near the target track until it becomes on-track, or on-track becomes difficult to achieve.
Waveforms of RF signals SA, SB, SC, and SD obtained from light receiving areas A, B, C, and D of the photo-detector (FIG. 12) during a track jump are shown schematically in FIG. 15. Sine-wave-like level change Ste appears at the bottom of the waveform of each RF signal because beam spot SP of laser beam LB irradiated from optical pickup 14 to signal recording plane 10a crosses a track part and an inter-track mirror part (flat part where no pits are present) alternately in the radial direction at a fixed speed. That is, when beam spot SP crosses the track part, intensity modulation or diffraction is maximized by pits (light intensity of the reflected beam is thus minimized), and the intensity modulation is minimized (light intensity of the reflected beam is thus maximized) when beam spot SP crosses the mirror part. Normally, a portion of beam spot SP overlays on adjoining tracks even when it is at the center of the mirror part, and the overlaid parts are subject to intensity modulation. Thus, the maximum point of the bottom level of an RF signal never becomes as high as the top level, and the high-frequency modulated element also remains near the maximum point.
In the case of the conventional tracking error detection circuit (FIG. 14), tracking error TE representing the positional error of beam spot SP in the radial direction can be obtained by applying prescribed operations (push-pull operations) using operation circuits (100, 102, and 104) to RF signals SA, SB, SC, and SD obtained from respective light receiving areas A, B, C, and D of the photo-detector. However, during low-speed playback, the frequencies of the RF signals and the frequency of the tracking signal elements are close to each other, so that it is difficult to separate the signals using the low-pass filter, resulting in a problem that RF signal elements RFn are likely to remain in tracking signal TE as shown in FIG. 16. On the other hand, during high-speed playback, although the signals can be separated using the low-pass filter because the frequencies of the RF signals and the frequency of the tracking signal element are not close to each other, there is a problem that the SN characteristic becomes deteriorated since tracking signal TE becomes attenuated drastically (approximately ½) by the low-pass filter as shown in FIG. 17.
In the case of a general optical disk, such as a CD (Compact Disc) and a DVD (Digital Versatile Disk), EFM (Eight to Fourteen Modulation) is utilized as a method for modulating data to be recorded on the tracks, and the lengths of pits are restricted to fall within the range of 3T–11T (T represents one bit length) so as to satisfy the 2, 10 RLL (Run-length Limited) code requirement, that is, the length of “0” continues 2 times or more but not more than 11 times. Therefore, although the channel clock frequency is 4.32 MHz when a CD is played back at 1× speed, the frequencies of the RF signals become maximum, that is, 720 kHz, at the location 3T/3T, and they fall down to 196 kHz or so at the location 11T/11T. It is desirable that such RF signal elements in the tracking error signal by small enough to be ignored in order to achieve tracking servo accurately.
However, in the case of the conventional tracking error detection circuit, it is usually difficult to separate or cut RF signal elements of 196 kHz or so relative to tracking error signal TE obtained from tracking modulation element Ste of 10 kHz even when they are put through low-pass filter 106.
In addition, during the conversion into a digital signal, the amplitude of tracking error signal TE is increased using operation circuits (100, 102, and 104) or a special gain control amplifier (not illustrated) in order to match the input range of an AD converter (not illustrated) of the latter stage. However, because the RF signal elements are also amplified, the gain cannot be increased sufficiently. In addition, because the RF signal elements are mixed, the digital signal is not very precise.
On the other hand, during a playback of a CD at 30× speed or a DVD at 6× speed or faster, bands for the RF signals reach 10 MHz or higher. Thus, when the bands for the operation circuits 100, 102, and 104 are only several MHz or so, those operation circuits function as low-pass filters. Thus, not only are the RF signal elements attenuated, but tracking modulation element Ste from which tracking error signal TE is obtained is also attenuated to ½, that is, reduced to one half, and the SN drops by 6 dB as a result.
Although the example pertains to a push-pull system utilizing a quadrant type photo-detector, similar problems are also found for conventional tracking error detection circuits in other methods.
The present invention was developed in light of such conventional problems, and its objective is to present a tracking error detector by which a tracking error signal from which high-frequency elements are cut sufficiently can be obtained.
Another objective of the present invention is to present a tracking error detector by which a tracking error signal with an excellent SN ratio can be obtained.
Yet another objective of the present invention is to present a tracking error detector by which a tracking error signal which can be digitized easily can be obtained.